Greenhill Formula ?

[cavedweller]

@Ballisticboy

thoughts on this i picked up on facebook?

Greenhill Formula
helps you find the right twist rate for your barrel to make sure the bullet stays stable and accurate. Here's the formula:
(C x D2) ÷ L = T
T: twist rate (in inches)
C: A constant (150 for speeds below 850 m/s, 180 for higher speeds)
D: Bullet diameter (in inches)
L: Bullet length (in inches)
How to use it
1. Measure the bullet length and diameter
2. Choose the constant C based on the bullet’s speed.
3. Plug the numbers into the formula to get the right twist rate.
Why use it?
This formula helps you pick the right ammo for your barrel or check if your gun works well with specific bullets. The better the twist rate matches, the more accurate your shots will be

 

[dgeesaman]

The reference to 850m/s tells you a lot about its sweet spot. Centerfire shooters know this formula is only approximate for its intended application; airguns are well out of its range.

The Miller stability formula is better than Greenhill but airguns are still well out of its domain.

 

[6gun]

Strelok can show you how stabilized a projectile is too. I will run numbers for different projectiles before I buy.

 

[JaceSpace1369]

Have y'all used this one?

Twist Rate Stability Calculator

Our newly updated Twist Rate Stability Calculator not only tells you what your SG is, but it also lets you know if you are reaching your optimized BC.

bergerbullets.com

 

[addison]

Where do you guys find information for fx barrels twist rates?

 

[WoodWelder]

Where do you guys find information for fx barrels twist rates?

Official FX STX Barrel Liner Twist Rates

There has been a lot of questions lately from shooters wanting to know the twist rate of their barrel. Here is an official list of twist rates for the current offering of FX STX barrel liners. Note: Some shooters have speculated that FX has changes twist rates on some of these liners...

www.airgunnation.com

 

[addison]

Good deal mines 1:18 .177 now to figure best ammo

 

[Ballisticboy]

All of the simple systems use large assumptions and simplified equations to calculate twist rates. The Greenhill formula was one of the first and is one of the least accurate. The Miller formula and later additions again make sweeping assumptions about the physical construction of the projectile and the aerodynamics. The Berger bullets method is based on the modified Miller formula. I do not know what the Strelok program uses, but the program does not have sufficient data input for an accurate calculation, such as the projectiles CG position, moments of inertia or aerodynamic moments coefficient.

The most accurate of the approximate methods is probably the Kolbe calculator, which is based on Bob McCoy's McGyro equations. However, these equations again are based on the assumption of a solid uniform shot with no base or nose cavities, which are not very common with slugs. No methods work for pellets, which require a lot more pain to calculate.

 

[cavedweller]

All of the simple systems use large assumptions and simplified equations to calculate twist rates. The Greenhill formula was one of the first and is one of the least accurate. The Miller formula and later additions again make sweeping assumptions about the physical construction of the projectile and the aerodynamics. The Berger bullets method is based on the modified Miller formula. I do not know what the Strelok program uses, but the program does not have sufficient data input for an accurate calculation, such as the projectiles CG position, moments of inertia or aerodynamic moments coefficient.

The most accurate of the approximate methods is probably the Kolbe calculator, which is based on Bob McCall's McGyro equations. However, these equations again are based on the assumption of a solid uniform shot with no base or nose cavities, which are not very common with slugs. No methods work for pellets, which require a lot more pain to calculate.

Thank you sir